This invention relates to a composite oxide having a novel crystal structure and exhibiting particularly excellent oxygen absorbing and desorbing capability which is utilizable for functional ceramics and for a catalyst for purifying exhaust gases, and a method for preparing the same.
Cerium oxide has hitherto been employed in large quantities as ceramics or as catalysts for purifying exhaust gases. In the field of catalysts, for example, cerium oxide is used for enhancing the purification rate of exhaust gas components such as hydrocarbons, carbon monoxide and NO.sub.x, taking advantage of its characteristic properties of absorbing oxygen in an oxidative atmosphere and desorbing oxygen in a reducing atmosphere. In the field of ceramics, cerium oxide is used in the form of compounds or mixtures with other elements as electrically conductive ceramics such as solid electrolytes, taking advantage of its characteristic properties mentioned above. Such cerium oxide heretofore known is usually prepared, for example by adding oxalic acid or ammonium bicarbonate to a solution of nitrate or chloride of cerium, filtering and washing the resultant precipitate and drying followed by calcining.
However, the composite oxide prepared by the above method predominantly composed of cerium oxide has a drawback that it is incapable of sufficiently absorbing and desorbing oxygen at 400.degree. to 700.degree. C., and is deteriorated in performance after heating to a higher temperature, even though it exhibits the oxygen absorbing and desorbing capability.
Up to now, cerium-zirconium composite oxide exhibiting high oxygen absorbing and desorbing capability is known and disclosed in Japanese Laid-Open Patent Application No. 5-105428 (1993). For preparing the composite oxide, there is known a method including mixing, into a solution containing cerium, zirconium and hafnium ions, a solution containing other metal ions and an aqueous solution of ammonia, ammonium bicarbonate or oxalic acid, to form a compound salt precipitate, and calcining the compound salt precipitate at a temperature not lower than 300.degree. C. However, the calcining temperature contemplated is not higher than 400.degree. C., while there is no disclosure as the crystal structure of the composite oxide having .PHI. phase. Detailed scrutiny into the crystal structure of such oxide by Comparative Examples given hereinbelow has revealed that the structure is a mixed phase of a CaF.sub.2 -related structure phase, an h-tetragonal phase and a monoclinic phase. On the other hand, the tetragonal .PHI. phase has been known as the crystal phase (V. Longo and D. Minichelli, J. Amer. Ceramic Soc., 56(1973), 600.; P. Duran, M. Gonzalez, C. Moure, J. R. Jurado and C. Pascual, J. Materials Sci., 25(1990), 5001). It is reported that this phase is formed after annealing at 993K for several months, and extremely difficult to manufacture. On the other hand, there has not so far been known a composite oxide containing cerium oxide, zirconium oxide, hafnium oxide and other metal oxides and having .PHI. phase.